Antonio is the merchant of venace and is in love with Bassanio. However, Bassanio is in love with Portia and asks Antonio for money to be able to marry her. Antonio tells Bassanio to visit Shylock to ask for a lone while Antonios merchanise is being shipped across the sea.
Shylock agrees to give Bassanio the loan but only if he gives it back. Shylock says that if he fails to give the money back then Antonia will be killed (Shylock and Antonio already have a bad relationship).